Attendance was very roughly 1500. This was close to the 50th anniversary of the First Convention held in Philadelphia in 1936 and post hoc adopted as the first Philcon, and there was significant program based on it as well as a good turnout of First Fandom. (And Jack Chalker reports that the many people were saying that after fifty years they still haven't gotten it right — a calumny he denies, noting that 1961 wasn't bad at all!)